The included Windows Defender app is actually a decent AV, and should be all you need if you aren't prone to going places on the internet where you can pick up malware. If you want a third party solution, look at suites like VIPRE or Norton AV or Internet Security. And contrary to what some may tell you, the last few iterations of the Norton line of security products are quite good. I've personally used them in both the beta stage, as well as after their official release.
As far as keeping your computer clean (i'm assuming in the physical sense, as in dust buildup and such), you're stuck with getting a case that promotes lots of airflow, and simple maintenance every 1-3 months, depending on the environment the computer is in. Dust happens, and there is nothing we can do to stop it.